Absorbent articles such as diapers;, incontinence products and feminine care products have generally involved some combination of an impervious backing material, an absorbent material, and a cover or liner material. The cover layer or liner layer should have desirable fluid-handling characteristics, yet be soft and comfortable for the wearer. In the past, a variety of cover materials have been used such as, for example, perforated films, netting materials and nonwoven webs. Although perforated films and netting generally have desirable fluid-handling characteristics, they have the disadvantage of feeling clammy and uncomfortable next to the skin. Conventionally formed nonwoven webs of melt-spun filaments have acceptable fluid handling characteristics and more desirable tactile characteristics than nettings or perforated films. The melt-spun filaments of such nonwoven webs can be crimped to further improve softness, visual and tactile aesthetics as well as fluid handling performance. More particularly, the decreased bulk and openness provided by crimped filaments is associated with improved fluid acquisition and transfer.
Crimped filaments may be made by several methods. Mechanical crimping techniques or bi-component or multi-component filaments are expensive and require special manufacturing processes. Fiber crimping caused by differential cooling of an un-solidified melt-spun filament is known. For example, U.S. Pat. No. 4,783,231 to Raley describes a nonwoven web containing fibers that have been differentially cooled to impart a crimped configuration. The fibers can be thermally relaxed to partially decrimp the fibers which increases the loft and decreases the density of the nonwoven web. According to that patent the fibers may be made of any material generally satisfactory for formation of fibers, such as, for example, polypropylene, polyethylene, polyester, nylon, rayon, polyurethane, cellulose and compatible blends of those materials.
However, melt-spun fibers of conventional homopolymers and polymer blends generally must be fine, low denier fibers in order to respond effectively to differential cooling. Nonwoven webs of such crimped, fine melt-spun filaments generally appear to have relatively poor fluid acquisition and transfer characteristics and may be difficult to process. Thicker filaments may be produced, however, they do not crimp very well and typically form nonwoven webs that have undesirable stiffness and tactile properties.
Crimped copolymer filaments are disclosed in, for example, U.S. Pat. No. 3,929,542 to Gehrig et al. That patent discloses nonwoven webs of helically crimped filaments formed from polyethylene and copolymers of ethylene with 1-olefins, vinyl esters, acrylic esters, and vinyl chloride. The proportion of co-monomer in the total ethylene copolymer may be up to about 30% by weight. It is also disclosed that crimped nonwoven webs may be formed of polypropylene and polybutene, copolymers of propylene and 1-butene with each other and with other 1-olefins. The proportion of co-monomer in the total propylene and/or 1-butene copolymer may be up to about 15% by weight. According to Gehrig et al., the crimped filaments are formed by extruding the molten polymer through a spinnerette and then directing cooling air against bundles of newly formed filaments. The filaments then enter a crimping device and are crimped by periodically applying and removing one or more annular or cylindrical vortices of a gaseous medium with partial flow-off and replenishment of the vortex or vortices. Although partial one-sided cooling is disclosed as assisting the crimping of the filaments, Gehrig et al., teaches that over-cooling is undesirable since the filaments can no longer be shaped in the crimping zone unless hot gas or steam is introduced. After leaving the crimping zone, Gehrig et al. discloses that the bundles of filaments pass through moveable elements (e.g., flexible tubes) to mouthpieces which are moved in a prescribed swinging motion so that the filaments are laid down in a perfect or imperfect trochoid pattern.
Thus, there exists a need for a cover material having a combination of softness and the high bulk/low density associated with desirable fluid transfer characteristics that can be produced easily and economically utilizing conventional filament forming apparatus without specialized crimping and/or web forming devices.